According to Mike Albers, forest health specialist for the Minnesota Department of Natural Resources: “Your spruce trees are not dying and it’s too late in the season to spray anything on the trees.”

Over the last month or so, many homeowners have noticed their spruce trees turning tan or orange. The cause is two types of spruce needle rust fungi, Chrysomyxa ledi and Chrysomyxa ledicola. Even though the fungi look bad, they are more of an aesthetic problem than a tree health problem. However, they can be tough on newly planted trees and Christmas trees. The rust is most common on blue spruce but can also infect white and black spruce.

The fungus only infects the current year needles, which will turn yellow and fall off the tree later this fall. The tree will not grow new needles in place of those that fall off. However, the tree will have healthy buds on the ends of the branches and these will produce new needles next year.

The rust does not infect the shoots or branches of the tree or older needles on the tree. Healthy trees survive the infection with little or no long-term damage. Most likely, homeowners will not see infection on their trees next year. “In some years, like this one, spruce needle rust is very common,” Albers said, “But in most years it’s hard to find.”

The fungus requires other plants such as Laborador tea or leather leaf to complete its life cycle. In early summer, the rust fungus produces spores on leaves of Laborador tea or leather leaf. If the wind blows these spores onto current year spruce needles and if the weather is wet and cool, the spruce needles become infected and turn yellow, orange or tan in July and August.

The rust fungus produces spores on infected spruce needles and these are carried by wind and rain splash to spread the infection to other Laborador tea or leather leaves. The spores produced on spruce trees do not infect other spruce trees.

Once spruce needle rust becomes obvious on the trees, it is too late to use a pesticide. When the needles start to turn color they are already infected and fungicides cannot cure the infected needles. Pesticides are seldom recommended because the infections usually don’t cause long-term damage, and because there is no way to predict during which years the rust will be a problem.

The best thing homeowners can do is to keep their trees well watered if the weather turns dry.

“Avoid using sprinklers, because they get the needles wet and lead to needle and twig disease problems such as needle rust. It is much better to lay the hose on the ground under the tree to soak the ground,” Albers said.

Other ways to help prevent the rust include keeping weeds and grass short around small trees so winds can dry the needles better and prevent infection. Mulching around yard trees keeps the soil moist and also helps keep the weeds and grass away. Mulching also keeps your lawn mower and weed whip away from your trees reducing injury to the stem. These are likely to do a lot more damage to your trees than spruce needle rust will.